The FAC signatures were also recorded by Polar at low altitudes ( 5000 km) near the Polar perigees in the southern hemisphere on both days. There were two perigee passes on these days, and the FACs were crossed twice for each pass, one in the pre-noon sector and one in the pre-midnight sector. In Figure 5, we compare the FAC magnetic signatures seen at same local times on the two days. Shown in Figure 5 are the magnetic field residuals of the east-west component as a function of magnetic latitude of Polar footprints. The thick traces are for May 11 and the thin traces for May 14. On the nightside about two hours before midnight, the FACs showed structure of well-defined thin current sheets on both days. The upward flowing current was at higher latitude (the region 1 current) and the downward flowing current sheet at lower latitude (the region 2 current), consistent with previously determined FAC pattern [Iijima and Potemra, 1976]. On the dayside pre-noon sector, the FAC magnetic field exhibited many fluctuations and complicated structures on both days, probably caused by the motion of FACs. We note that there are no significant differences on the strength of FACs observed for different solar wind conditions on these two days. Thus, the solar wind dynamic pressure only plays a minor role, if any. The IMF Bz component is the major controlling factor of the strength of field-aligned currents. The same conclusion has been reached based on observations under very high solar wind dynamic pressure during a magnetic cloud event [Le et al., 1998].